The tasks carried out by Hudswell Clarke 0-6-0ST No.1539 “Derek Crouch” during its decades in service included working at Tilbury Docks in Essex and transporting freight from farms and factories on the Wissington Light Railway in Norfolk.

The NVR team had been searching for the missing piece for decades, but said whoever took it may have done them a favour.

If they had not kept such good care of it, the part could have been damaged, or lost.

Mr Wilson told BBC Radio Cambridgeshire: “It is fantastic, it gave me a really good laugh when the letter appeared on the desk.

“We know nothing about who it was from, but we are grateful they looked after it and returned it to us.

“We have got no hard feelings.

“I’ve been involved in looking after the engine for about 12 years now and we have never had it… we presumed it was long gone.”

The letter from the culprit began: “… yes it is your original, I’m afraid you lost it in the 1980s.”

It concluded: “Good luck with your restoration, please find enclosed a small additional contribution.”